Magma Pressure-Temperature-Time Paths During Mafic Explosive Eruptions
Magma Pressure-Temperature-Time Paths During Mafic Explosive Eruptions
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DOI：
10.3389/feart.2020.531911
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发表时间：
2020-09
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影响因子：
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通讯作者：
M. Newcombe;T. Plank;Youxue Zhang;M. Holycross;A. Barth;A. S. Lloyd;D. Ferguson;B. Houghton
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文献类型：
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作者：
M. Newcombe;T. Plank;Youxue Zhang;M. Holycross;A. Barth;A. S. Lloyd;D. Ferguson;B. Houghton
我们约束同生喷发的压力-温度-时间（P-T-T）的路径的镁铁质岩浆使用的组合，短时间尺度的冷却和减压计时器。最近的工作表明，在喷发的最后几秒钟到几个小时的晶体的热历史可以约束使用浓度梯度内的MgO橄榄石托管熔融包裹体，在响应syneruptive冷却和结晶的橄榄石上的包体壁。我们已将这一技术应用于弧火山和洋岛火山喷发的熔融包裹体的研究，包括1974年Fuego火山的subplinian喷发，1977年Seguam火山的火喷泉喷发，1978年Seguam火山的火山喷发，1979年Seguam火山的火山喷发。以及基拉韦厄火山的三次喷发（1959年基拉韦厄伊基火喷泉喷发的第一幕、公元1500年的强烈火喷泉喷发和公元1650年的次subplinian喷发）。在迄今为止研究的喷发中，1959年基拉韦厄伊基喷发的熔融包裹体记录了最高的同喷发冷却速率（3-11°C/s）和最短的冷却持续时间（4-19秒），而1974年火地岛喷发的包裹体记录了最慢的冷却速率（0.1-1.7°C/s）和最长的冷却持续时间（21-368秒）。推断的基拉韦厄伊基和Seguam火喷泉喷发的高冷却速率与碎裂和喷发期间和之后数十秒的空气淬火是一致的。在同一次喷发中，从火地岛小型火山弹（直径1.6厘米）内部取样的熔融包裹体比从火山灰（粒径< 2毫米）取样的包裹体冷却得更慢，正如基于传导冷却模型所预期的那样。我们发现的证据冷却速率和减压速率的岩浆，其中快速上升的含气岩浆经历缓慢的冷却过程中的上升和喷发比缓慢上升的岩浆之间的系统关系。我们的岩浆P-T-T约束的基拉韦厄伊基喷发是在广泛的协议与等熵模型表明，在管道中的冷却的主要驱动程序是绝热膨胀的蒸汽相，但是，在Fuego和Seguam，我们的研究结果表明潜热生产和/或开放系统脱气（这两个违反假设所需的等熵上升）的重要作用。因此，我们警告不要将等熵导管模型应用于含有相对较高初始水浓度的岩浆（例如，含1.4wt%水的弧岩浆）。我们注意到，几个过程，已被推断发生在火山管道，如岩浆失速，岩浆混合，开放和封闭系统脱气，蒸汽通量，蒸汽积累（在泡沫层或作为蛞蝓的气体）与不同的隐含蒸汽体积分数在同生喷发上升。考虑到岩浆P-T-T路径对蒸汽体积分数的敏感性，这里提出的同辉温度计可能是在镁铁质岩浆喷发前几秒钟到几小时内识别这些过程的一种手段。
We have constrained syneruptive pressure-temperature-time (P-T-t) paths of mafic magmas using a combination of short-timescale cooling and decompression chronometers. Recent work has shown that the thermal histories of crystals in the last few seconds to hours of eruption can be constrained using concentration gradients of MgO inside olivine-hosted melt inclusions, produced in response to syneruptive cooling and crystallization of olivine on the inclusion walls. We have applied this technique to the study of melt inclusions erupted by arc and ocean island volcanoes, including the 1974 subplinian eruption of Fuego volcano; the 1977 fire-fountain eruption of Seguam volcano; and three eruptions of Kilauea volcano (episode 1 of the 1959 Kilauea Iki fire-fountain eruption, the 1500 CE vigorous fire-fountain eruption, and the 1650 CE subplinian eruption). Of the eruptions studied so far, melt inclusions from the 1959 Kilauea Iki eruption record the highest syneruptive cooling rates (3–11°C/s) and the shortest cooling durations (4–19 s), while inclusions from the 1974 Fuego eruption record the slowest cooling rates (0.1–1.7°C/s) and longest cooling durations (21–368 s). The high cooling rates inferred for the Kilauea Iki and Seguam fire fountain eruptions are consistent with air quenching over tens of seconds during and after fragmentation and eruption. Melt inclusions sampled from the interiors of small (∼6 cm diameter) volcanic bombs at Fuego are found to have cooled more slowly on average than inclusions sampled from ash (with particle diameters < 2 mm) during the same eruption, as expected based on conductive cooling models. We find evidence for a systematic relationship between cooling rates and decompression rates of magmas, in which rapidly ascending gas-bearing magmas experience slower cooling during ascent and eruption than slowly ascending magmas. Our magma P-T-t constraints for the Kilauea Iki eruption are in broad agreement with isentropic models that show that the dominant driver of cooling in the conduit is adiabatic expansion of a vapor phase; however, at Fuego and Seguam, our results suggest a significant role for latent heat production and/or open-system degassing (both of which violate assumptions required for isentropic ascent). We thereby caution against the application of isentropic conduit models to magmas containing relatively high initial water concentrations (e.g., arc magmas containing ∼4 wt% water). We note that several processes that have been inferred to occur in volcanic conduits such as magma stalling, magma mingling, open- and closed-system degassing, vapor fluxing, and vapor accumulation (in foam layers or as slugs of gas) are associated with different implied vapor volume fractions during syneruptive ascent. Given the sensitivity of magma P-T-t paths to vapor volume fraction, the syneruptive thermometer presented here may be a means of identifying these processes during the seconds to hours preceding the eruption of mafic magmas.